This relates to graphics processing.
In graphics processing units there is a rasterization unit or rasterizer responsible for finding which samples are inside the primitives being rendered. The rendered primitives are typically triangles.
Before a triangle is rasterized, there is usually a setup and clip test that determines whether the triangles are outside the view frustum. If so, those triangles can be culled, saving processing cycles. Then only the unculled triangles reach the actual rasterizer.
One culling test, called a coverage test, determines whether a triangle covers any samples at all given the current active sample pattern in the rasterizer. If it does not, then that triangle can be culled. A simple example is a triangle with two vertices having the same position, as it has zero area. Another is a triangle just falling in between samples.
For every sample within each covered pixel, the rasterizer executes a coverage test by evaluating if the sample is inside the primitive. For a triangle, this is done by evaluating the signed distance of the sample to the three edges of the triangle.